Gratified to see that the focus of the investigation is going to the battery itself, which IMO was always the most likely suspect. It will be interesting to see what cooling measures were adopted, because it seems unlikely that this design requirement was simply ignored.
Also, before the press get all wound up about cooling, note that only two batteries, out of all the batteries in all the 787s out there, had this heat stress problem. Was the cooling design too close to the margin? Or was there something anomalous about some of the battery packs? It looks like some lithium-ion variants are more prone to overheating than others, for example.
My bet continues to be on the battery design itself. However this does not preclude taking other steps, to make even the more overheating-prone samples of such batteries safe to use.

Pictures of the battery make it appear that there were no cooling fans, no active liquid coolers, no heat pipes or even heat sinks in the battery design, only a 1/2" air channel around each cell to provide convective coupling to the metal battery case.
This is very interesting considering the battery packs were designed for high discharge/charge rates.
Elon Musk, who knows a bit about this, said the 787 battery design was incompetent and dangerous.

Has anyone considered the lack of adequate convective cooling at cruising altitude? I used to work at Los Alamos in the 80s and several devices designed at sea level would fail due to overheating at the 7700 feet altitude arising from the lower air density, especially CRT computer terminals.